It called on the UN Security Council to ask the International Criminal Court to investigate a security sweep in Talkalakh where residents told of sectarian killings, gunmen carrying out execution-style murders and the stench of decomposing bodies in the streets.

Some activists have said the death toll from the May siege was as high as 36 people.

"The accounts we have heard from witnesses to events in (Talkalakh) paint a deeply disturbing picture of systematic, targeted abuses to crush dissent," said Philip Luther, Amnesty International's Middle East and North Africa Deputy Director.

The report said the attacks "appear to be part of a widespread, as well as systematic, attack against the civilian population," which would constitute crimes against humanity.

Talkalakh is just across the border from Lebanon.

Amnesty quoted witnesses as saying Syrian forces fired on fleeing families and ambulances carrying the wounded; one witness said soldiers stabbed lit cigarettes on the backs of detainees' necks.

At least nine people died in custody, witnesses told Amnesty. Eight of them were shot at and wounded as they were ordered out of a house, and were then taken away by soldiers.

Amnesty cited interviews carried out in Lebanon and by phone with more than 50 people. The rights group, along with most foreign media, has not been allowed to enter Syria.

The 14-week uprising against president Bashar Assad has proved resilient despite a deadly government crackdown that has brought international condemnation and sanctions. Assad is facing the most serious challenge to his family's four decades of rule in Syria.